Okay, so you can do this.
Change your links to be this format
At the start of your page put this in
Then use the variable $cat in your sql statment to get the results you want displayed.
$url = $_SERVER['REQUEST_URI'];
//from the address get the categorey
//Not sure how $url will be if it will have the and symbol or the &
// so by result of this echo if it is & change the & below to that
$page = explode ("forum.php?&cat=", $url);
$cat = $page;
You might want to initialize $cat as something or use an if statment around your sql so it isn't triggered when $ is empty or not set.